Currently, the electricity industry is facing unprecedented challenges. To accelerate the pace of development of electric power, it is faced with the power to accelerate the pace of structural adjustment. The most realistic and feasible way is to accelerate the construction of ultra-supercritical units. Supercritical pressure parameter is the effective use of thermal energy in a new technology. Through its steam pressure and temperature increased to some extent, to improve the thermal efficiency of the unit. To ensure that the unit at a higher temperature and pressure stable operation, it is necessary to use a higher temperature strength of steel, or the wall thickness of the member must be increased exponentially to meet service requirements. Currently the temperature of P92 steel used in the boiler main steam was raised to 610° C., reheat temperatures up to 625° C. P92 steel has been widely used in the USC units set box and main steam piping and other components. P92 steel was used widely in China without of foreign experience.
On economic and technological grounds, P92 pipes are usually fabricated from separate pieces by welding girth joints. However, after welding process the joints are marked by residual welding stress and nonuniform microstructures and properties, which will impair the operational reliability of components. Hence, PWHT should be instantly carried out. For large-sized vessels and apparatuses, placing the entire component in a furnace or oven is impractical. In such case, the weld metal, heat-affected zone (HAZ), and a limited volume of base metal adjacent to the weld might be subjected to heating using the local heat treatment (LHT) technology. Compared with the overall heat treatment, it saves fuel and material resources substantially. The primary benefit of local PWHT is relaxation of residual stresses. Other significant benefits are reducing susceptibility to brittle fracture and improving notch toughness of welded joints. Heated band consists of the soak band plus adjacent base metal over which the heat source is applied to. Because of the inherent radial temperature gradient, the band should be large enough to achieve the required temperature and limit the induced stresses within the soak band. ASME Boiler and Pressure Vessel Code, Rules for Construction of Pressure Vessels: Alternative Rules, Section VIII, Division 2 As a minimum, the soak band shall contain the weld, heat affected zone, and a portion of base metal adjacent to the weld being heat treated. The minimum width of this volume is the widest width of weld plus the nominal thickness defined in paragraph 6.4.2.7 or 50 mm (2 in.), whichever is less, on each side or end of the weld. The equation of Heated Band (HB) width and Gradient Control Band (GCB) width has not provided. In summary, various standards or codes have different methods to define the HB width and GCB width. Hence it may bring confusion in engineering practice.
Results of the literature search indicate that no study has been conducted to take the field conditions during local PWHT of P92 welded joints into consideration. Under field conditions, effect of local PWHT may be influenced by air convection situation, material types, insulation condition, et al. and determination of the local PWHT criterion for ASME P92 pipes becomes a more complicated problem.
Currently many welding joints cracks of P92 pipe was observed in many service after LHT. It should be inappropriate for local heat treatment process. Therefore, it is significant to choose the right LHT technology for the protection of local heat treatment P92 safe operation of pipelines.